NOTE: As of April 2009, all content sold at the iTunes Store is in iTunes Plus format. The option to upgrade now works differently than demonstrated in this article. To see what songs you can upgrade to iTunes Plus click here. (Thanks to reader Øyvind for the tip!)
Music purchased at the iTunes Store prior to Jan. 2009 can be upgraded to the iTunes Plus DRM -free file formats (all music bought after Jan. 2009 is automatically iTunes Plus content).
With millions of songs eligible, so it's likely that you've got some music bought at the iTunes Store that can be upgraded. Since upgrades are only US$0.30 per song or 30% of the cost of an album, that option is even more appealing.
Here's a step-by-step guide to upgrading the music you've bought at the iTunes Store to the iTunes Plus format.
First, start by enabling iTunes Plus.